1. Field of the Invention
The present invention relates to an anisotropic porous material including pores having an orientation.
2. Description of the Prior Art
General porous materials are roughly classified into three typical types shown in FIGS. 1A to 1C. A first type is called a sinter-type porous material. As shown in FIG. 1A, the sinter-type porous material is formed by bonding and solidifying solid particles 1 at particle contact points. Pores 2 are formed of gaps among the solid particles 1. A second type is called a foam-type porous material. As shown in FIG. 1B, Pores 4 are formed of a partition made of a solid material 3. A third type is called a sinter-and-foam mixed phase type porous material, which is made of a mixed phase material composed of a sinter-type porous material and a foam-type porous material. As shown in FIG. 1C, the sinter-and-foam mixed phase type porous material contains pores 5 each having a form of the sinter-type porous material and pores 6 each having a form of the foam-type porous material.
These porous materials are used for a wide range of purposes, such as a filter for filtering various fluids in both a gaseous phase and a liquid phase, a heat insulator, an acoustic absorbent, and a shock absorbent. As an example, the present state of a porous membrane used for the purpose of water purification will be described in detail below.
In a water purifying plant, raw water is taken from water sources such as a river and a reservoir. Then, the raw water is treated in five unit processes including coagulation, flocculation, precipitation, filtration, and disinfection. Thereby, suspended materials and colloid materials are removed, and bacteria and the like are made harmless. Thus, clear tap water is supplied to consumers.
A method using coagulants is generally used in a series of clarifying processes by means of the coagulation, the flocculation, the precipitation and the filtration. Inorganic metal salts such as iron and aluminum are usually used as the coagulants. Effects of the coagulants are affected by various physical and biochemical factors. Thus, the optimal coagulation condition can be established only when a complex equilibrium is reached among many factors. For this reason, skill is needed to secure a certain level of treated water quality.
In October 1996, the Ministry of Health and Welfare (currently, called the Ministry of Health, Labour and Welfare) issued the “Policies concerning Temporary Measures against Cryptosporidiums Contained in Tap Water.” The policies establish a regulation instructing to always read water turbidity at a filter bed outlet, and thus to keep the water turbidity at the filter bed outlet at 0.1 degree or under. Accordingly, management of the water turbidity in the water purifying plants has become a significant issue.
Against this background, research and development concerning microfiltration membranes and ultrafiltration membranes have been advanced. In Japan, membrane filtration has started to be rapidly widespread in water purifying plants. In other countries, membrane filtration water purifying plants have been already in operation, and each of the plants treats several hundreds of thousand tons of water per day. The membrane filtration using the microfiltration membrane or the ultrafiltration membrane has an advantage that good treated water quality is obtained by surely removing turbid substances.
Organic polymer membranes (for example, cellulose acetate, polysulfone, polyethylene, polypropylene and polyacrytonitrile) have been most widely used as materials for the microfiltration membrane and the ultrafiltration membrane. However, the life of the organic polymer membranes is three to five years due to performance deterioration caused by a change of properties of the membranes themselves, and due to performance deterioration caused by external factors, as its operation time increases. The change of properties of the membranes is caused by: physical deterioration such as compaction and damage of the membrane; chemical deterioration such as hydrolysis and oxidation of the membrane; biological deterioration resulting from membrane utilization by microbes; and the like. An example of the external factors is accumulation of fine particles and suspended materials on the membrane surface. Thus, the membrane filtration using the organic polymer membranes has a disadvantage of high running costs due to costs needed to exchange the membranes.
Japanese published unexamined application No. 2001-225057 discloses a technique to reduce such running costs. This technique is a water treatment system surely removing fine particles and suspended materials in the following way. First, coagulated flocs are formed by use of coagulants, and then are removed by sand filtration. Subsequently, the fine particles and the suspended materials are removed by a metal membrane filtration apparatus that is excellent in durability.
The metal membrane filtration apparatus disclosed in Japanese published unexamined application No. 2001-225057 is configured of a cylindrical element. The cylindrical element is formed of a pleated metal membrane in a nonwoven fabric state, which is obtained by sintering stacked metal fibers.
On the other hand, as a filtration membrane other than a metal membrane, porous ceramic membranes obtained by sintering fine particles are disclosed in Japanese published unexamined applications No. 2001-259324, No. 2001-259323, No. Hei 10(1998)-236887, and No. Hei 10(1998)-235172.
There are the following problems in the metal membrane filtration apparatus disclosed in Japanese published unexamined application No. 2001-225057.
(1) Reduction of Flux Due to a Nonwoven Fabric Structure
A metal membrane with a nonwoven fabric structure has a structure to capture fine particles and suspended materials not only on the surface of the metal membrane but also inside the metal membrane. Accordingly, the metal membrane has an advantage of being capable of capturing, in the inside of the membrane, the fine particles and suspended materials that cannot be captured on the surface of the metal. On the other hand, it is impossible to remove the fine particles and the suspended materials that have penetrated into the membrane by normal cleaning. Thus, the reduction of flux is facilitated, as its operation time increases.
(2) Pollution Concern Due to Adding Coagulants and Increase of the Amount of Process Matters Due to Flocculation
As described above, in the case of the metal membrane, it is difficult to remove the fine particles and suspended materials that have penetrated into the membrane. For this reason, it is inevitable to perform a process of removing removable suspended substances by flocculation as much as possible in advance. This process raises a concern of pollution caused by injecting chemicals, that is, adding coagulants. Moreover, since it is necessary to waste the flocs, the amount of process matters increases.
In addition to the nonwoven fabric metal membrane obtained by sintering stacked metal fibers, a porous metal membrane obtained by sintering metal fine powder has been studied, but it also has the same problems as described above.
On the other hand, regarding to the ceramic membranes disclosed in Japanese published unexamined applications No. 2001-259324, No. 2001-259323, No. Hei 10(1998)-236887, and No. Hei 10(1998)-235172, there is a report that it is possible to form pores each having a finer diameter than that of the metal membrane, and to provide an excellent backwash. As is the case with the metal membrane described above, however, the ceramic membrane has a structure to capture the fine particles and the suspended materials not only on the surface of the membrane but also in the inside thereof, since the ceramic membrane is basically a porous body obtained by sintering fine particles in a network form. For this reason, there are also problems that it is difficult to remove the fine particles and suspended materials that have penetrated into the membrane, and that the reduction of flux is facilitated as the operation time increases. In addition, since the ceramic membrane has a structure in which pores form a complex network, pressure loss is relatively large even in initial properties.